One possibility for transcutaneously providing an implanted stimulator with power and information is to transmit rf-power via an inductively-coupled coil system. Such a coil system consists of a primary coil which is outside the body and a secondary coil implanted with the stimulator. When facing each other, the primary and secondary coils form a transformer which allows energy transfer from a transmitter powering the primary coil to the implanted stimulator. For example, cochlear prostheses systems utilize inductively coupled coils for power and data transmission. The diameters of primary and secondary coils for such applications are typically between 15 and 30 mm, and the coil separation is typically between 4 and 15 mm.
The minimum geometric size of the coils in general is determined by the distance between the coils. To achieve a high amount of inductive coupling, the distance between the coils has to be sufficiently small compared to the coils' diameters. A sufficiently high inductive coupling is a necessary condition to achieve a high power transfer efficiency. If the coupling is too low, a higher current in the primary coil has to be used to provide the same output from the secondary coil. Thus more power is wasted in the primary coil due to ohmic losses.